In general, laminated glass composed of a pair of glass panels and a resin film interposed between these panels is excellent in safety, since when it is broken, the shattered pieces thereof do not scatter. Thus, it is widely used as window glass of vehicles such as automobiles and as that of buildings.
It is required that such laminated glass should have not only the capability of preventing shattered glass pieces from being scattered but also the capability of sound insulation. The capability of sound insulation is represented as a transmission loss that depends on frequency. Specifically, JIS A 4708 specifies sound insulation ratings according to the transmission loss at an arbitrary frequency of 500 Hz or more.
Although conventional laminated glass has a good capability of preventing glass pieces from scattering, it has a problem that the sound insulation performance is deteriorated due to the coincident effect in the frequency region of around 2,000 Hz. Thus, an improvement is demanded. The coincidence effect refers to a phenomenon that, when a sound wave is incident on a glass plate, a transverse wave propagates along the surface of the glass plate due to the rigidity and inertia of the glass plate, and the resonance between the transverse wave and the incident sound wave causes transmission of sound.
In the meantime, according to the equivalent loudness curve, the human ears have a significantly higher sensitivity at a frequency in the range of 1,000 to 6,000 Hz as compared with other ranges of frequency. Therefore, it is important in the improvement of sound insulation to prevent the deterioration in the sound insulation performance caused by the coincidence effect.
In order to improve the sound insulation performance of laminated glass, it is necessary to prevent a deterioration of the sound insulation performance in the minimum region of the transmission loss caused by the coincidence effect. In order to prevent a deterioration of the sound insulation performance in the minimum region of the transmission loss, a variety of measures have been proposed such as an increase in the weight of laminated glass, a construction of laminated glass composed of multilayers, a division of the area of laminated glass, and an improvement in the support structure of laminated glass. However, these measures are not sufficiently effective, and their commercial values are low due to an increase in the manufacturing cost.
Meanwhile, the criteria for sound insulation performance have been increasingly strict in recent years. In particular, in the case of windowpanes for architectural use, a high performance of sound insulation near room temperature is required. Thus, it is required that laminated glass have a good capability of sound insulation even when the ambient temperature changes over a wide range from a low-temperature region to a high-temperature region.
However, the conventional laminated glass manufactured using a resin film made of a plasticized polyvinyl acetal resin has a problem that the temperature for maximum sound insulation performance is room temperature or higher, and the sound insulation performance around room temperature is poor. In addition, although attempts have been made to secure a good performance of sound insulation, the resin film is too soft so that such a problem as panel shearing, foaming, and the like tends to occur when the resin film is combined with glass panels in the manufacture of laminated glass.
Specifically, Japanese Laid-open Patent Publication No. Hei 02-229742 discloses a polymer film having a glass transition temperature of 15° C. or less, for example, a polymer film composed of a laminate of a vinyl chloride-ethylene-glycidyl methacrylate copolymer film and a plasticized polyvinyl acetal film. This polymer film, however, is incapable of exhibiting a sound insulation rating of Ts-35 or higher in the sound insulation ratings according to JIS A 4706. It also has a limited temperature range for sound insulation, failing to exhibit a satisfactory performance of sound insulation over a wide range of temperatures.
Further, Japanese Laid-open Patent Publication No. Sho 51-106190 discloses a construction produced by laminating two or more kinds of resins having different glass transition temperatures so that it has a vibration damping property over a wide range of temperatures. However, it is not apparent that the construction has such properties required for laminated glass as sound insulation, transparency, and the like. It also fails to satisfy the requirements for safety glass such as a high absorption of impact energy, scattering prevention upon glass breakage, and the like.